1. Field of the Invention
The present invention relates to combustion control devices for performing opening and closing controls of gas feed passages to gas burners to control combustion, and particularly relates to combustion control devices which perform self-checking of safety circuits of gas burners with use of microcomputers prior to start of combustion operations of the gas burners contained in gas water heaters and the other gas appliances.
2. Description of the Related Art
As is known from, for example, Japanese Patent Laid-open No. 2000-161661, a gas appliance is provided with an on-off valve at a gas feed passage for supplying a gas to a gas burner to perform a combustion control. Recently, a gas appliance, which uses an electromagnetic type on-off valve as this on-off valve and performs an on-off control of the electromagnetic type on-off valve by a microcomputer, comes on the market. In the gas appliance of a microcomputer control, the electromagnetic type on-off valve is opened and an ignition plug is operated when a gas burner is ignited by an ignition plug or the like. A detection signal from a flame, detection circuit using a thermocouple or a flame rod which is placed in the vicinity of a burner port of the gas burner is detected with the microcomputer, and when it is determined that the ignition operation normally functions, the electromagnetic type on-off valve is controlled to be opened continuously.
When ignition is not confirmed even a predetermined time elapses after the electromagnetic type on-off valve is opened by an ignition start command from the microcomputer and the ignition plug is operated, the microcomputer determines that the ignition operation fails, and controls the electromagnetic type on-off valve to be closed.
However, if the microcomputer does not operate normally for some reason, there arises the possibility that the electromagnetic type on-off valve is not closed even when the ignition operation ends in failure. Thus, a safety circuit, which is safety means, is constituted such that it is provided separately from the microcomputer, and even if the microcomputer does not operate normally, the electromagnetic type on-off valve is compulsively closed unless the gas burner is ignited at the point of time at which the predetermined time elapses from the start of the ignition operation.
This safety circuit generates a misfire signal by combining the flame detection signal from the flame detection circuit and a detection signal of an on-off state of the electromagnetic type on-off valve provided at the gas feed passage. A switch such as a transistor is provided at a solenoid coil of the electromagnetic type on-off valve or a solenoid coil of a relay for energizing a solenoid, and when the misfire signal is outputted from the safety circuit, the switch is turned off to cut off passage of current to the solenoid coil of the electromagnetic type on-off valve compulsorily. The electromagnetic on-off valve is constituted so that the valve body is shifted to a closed valve state by a biasing force of a spring contained therein, when the passage of the current to this solenoid coil is cut off.
If the gas burner is not ignited at a point of time at which a predetermined time elapses after the microcomputer starts the ignition operation, the microcomputer compulsorily closes the on-off valve, and the predetermined time elapsing until the above-described safety circuit operates is set to be a longer time than the predetermined time provided for the microcomputer to confirm the ignited state.
The above-described safety circuit is provided for the case in which the microcomputer fails, but in the state in which the microcomputer normally operates, the safety circuit does not operate. Even if the safety circuit fails, the gas burner is ignited without any problems while the microcomputer is normally operating. Consequently, if the failure of the safety circuit is overlooked when the safety circuit fails, the safety circuit does not operate if the microcomputer fails, and there arises the problem that nothing is changed from the state in which the safety circuit is not provided.
Consequently, the present invention is made in view of the above-described problem, and has its object to provide a combustion control device capable of detecting a failure of a safety circuit.